A speaker is a device for radiating sound by generating compression-rarefaction waves in the air through transformation of electrical signals into vibration of a diaphragm. The principle of vibration of a diaphragm included in a speaker uses a phenomenon in which a low- or high-pitched sound is generated by vibrating the diaphragm at a low or high frequency using Lorentz force caused by a current flowing through a voice coil placed in a magnetic field. As the amplitude increases even at a frequency, the sound gets louder due to increased sound pressure.
Speakers include cone type speakers, plate type speakers, dome type speakers, horn type speakers, and the like. Among them, the plate type speakers are advantageous over other type speakers in that they can be manufactured in compact sizes by attaching voice coil plates directly to flat diaphragms.
FIG. 9 is a schematic sectional view of a conventional plate type speaker, and FIG. 10 is an exploded perspective view showing a coupling structure of a voice coil, permanent magnets and plates employed in the conventional plate type speaker.
As shown in FIGS. 9 and 10, a flat coil plate 36 with a top end to which a flat diaphragm 30 is vertically fixed is inserted into an assembly of upper plates 31a, 31b, 31c, 31d, 32a, 32b, 32c and 32d, lower plates 33a, 33b, 33c, 33d, 34a, 34b, 34c and 34d, and permanent magnets 37 fixed therebetween. The assembly of permanent magnets and upper and lower plates is fixed to a base frame 35. Then, an edge 39 is bonded to the periphery of the flat diaphragm 30 and secured by a guide ring.
An electric current generated at a speaker driving circuit is supplied to a voice coil 38 fixed at the coil plate 36 to produce a magnetic field at the voice coil 38. The magnetic field cooperates with a facing magnetic field generated from the polarity of the permanent magnet-plate assembly, thereby generating a force that vibrates the coil plate 36 and the flat diaphragm 30 fixed thereto. Consequently, the vibration of the diaphragm 30 causes sound to radiate outside.
However, in the conventional plate type speaker constructed as above, there are limitations on manufacture of slim speakers suitable for small appliances since the coil plate 36 is attached to a bottom end of the diaphragm 30 and the assembly of permanent magnets and upper and lower plates is formed and placed below the diaphragm 30.